Aluminum-copper-lithium alloy having improved compressive strength and improved toughness

ABSTRACT

The invention relates to a product based on an aluminium alloy comprising, as percentages by weight, 4.0 to 4.6% by weight of Cu, 0.7 to 1.2% by weight of Li, 0.5 to 0.65% by weight of Mg, 0.10 to 0.20% by weight of Zr, 0.15 to 0.30% by weight of Ag, 0.25 to 0.45% by weight of Zn, 0.05 to 0.35% by weight of Mn, at most 0.20% by weight of Fe+Si, at least one element selected from Cr, Sc, Hf, V and Ti, the amount of said element, if selected, being from 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weight for Hf and for V and 0.01 to 0.15% by weight for Ti, the other elements being at most 0.05% by weight each and 0.15% by weight in total, the remainder being aluminium. The invention also relates to a method for obtaining such a product and to the use thereof as an aircraft structural element.

FIELD OF THE INVENTION

The invention relates to products made of aluminum-copper-lithiumalloys, more particularly, such products intended for aeronautical andaerospace construction.

PRIOR ART

Aluminum alloy products are developed to produce high strength partsintended in particular for the aircraft industry and the aerospaceindustry.

Aluminum alloys containing lithium are of great interest in this regard,as lithium can reduce the density of aluminum by 3% and increase themodulus of elasticity by 6% for each weight percent lithium added. Forthese alloys to be selected in aircrafts, their performance in relationto other properties of use must reach that of commonly used alloys, inparticular in terms of compromise between the properties of staticmechanical strength (tensile and compressive yield strength, ultimatetensile strength) and damage tolerance properties (toughness, resistanceto the fatigue crack propagation), these properties being generallymutually exclusive. For some parts such as the upper wing skin, thecompressive yield strength is an essential property. These mechanicalproperties should moreover preferably be stable over time and have goodthermal stability, that is to say not be significantly modified by agingat operating temperature.

These alloys must also have sufficient corrosion resistance, be able tobe shaped according to the usual methods and have low residual stressesso that they can be fully machined. Finally, they must be able to beobtained by robust manufacturing methods, in particular, the propertiesmust be able to be obtained on industrial tools for which it isdifficult to guarantee temperature homogeneity within a few degrees forlarge parts.

U.S. Pat. No. 5,032,359 describes a large family ofaluminum-copper-lithium alloys wherein the addition of magnesium andsilver, in particular between 0.3 and 0.5 percent by weight, allows toincrease the mechanical strength.

U.S. Pat. No. 5,455,003 describes a method for manufacturing Al—Cu—Lialloys which have improved mechanical strength and improved toughness atcryogenic temperature, in particular thanks to suitable work hardeningand ageing. This patent recommends in particular the composition, inpercentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6 andZn=0-0.75.

U.S. Pat. No. 7,438,772 describes alloys comprising, in weightpercentage, Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourages the use ofhigher lithium content due to degradation of the compromise betweentoughness and mechanical strength.

U.S. Pat. No. 7,229,509 describes an alloy comprising (% by weight):(2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn,0.4 max Zr or other grain refiner agents such as Cr, Ti, Hf, Sc, V.

Patent application US 2009/142222 A1 describes alloys comprising (in %by weight), 3.4 to 4.2% of Cu, 0.9 to 1.4% of Li, 0.3 to 0.7% of Ag, 0.1to 0.6% of Mg, 0.2 to 0.8% of Zn, 0.1 to 0.6% of Mn and 0.01 to 0.6% ofat least one element for controlling the granular structure. Thisapplication also describes a method for manufacturing extruded products.

Patent application WO2009/036953 relates to an aluminum alloy productfor structural elements having a chemical composition comprising, byweight Cu from 3.4 to 5.0, Li from 0.9 to 1.7, Mg from 0.2 to 0.8, Agfrom about 0.1 to 0.8, Mn from 0.1 to 0.9, Zn up to 1.5, and one or moreelements selected from the group consisting of: (Zr about 0.05 to 0.3,Cr 0.05 to 0.3, Ti about 0.03 to 0.3, Sc about 0.05 to 0.4, Hf about0.05 to 0.4), Fe<0.15, Si<0.5, normal and unavoidable impurities.

Patent application WO 2012/085359 A2 relates to a method formanufacturing rolled products made of an aluminum-based alloy comprising4.2 to 4.6% by weight of Cu, 0.8 to 1.30% by weight of Li, 0.3 to 0.8%by weight of Mg, 0.05 to 0.18% by weight of Zr, 0.05 to 0.4% by weightof Ag, 0.0 to 0.5% by weight of Mn, at most 0.20% by weight of Fe+Si,less than 0.20% by weight of Zn, at least one element selected from Cr,Se, Hf and Ti, the amount of said element, if selected, being 0.05 to0.3% by weight for Cr and for Se, 0.05 to 0.5% by weight for Hf and from0.01 to 0.15% by weight for Ti, the other elements at most 0.05% byweight each and 0.15% by weight in total, the remainder being aluminum,comprising the steps of preparation, casting, homogenization, rollingwith a temperature greater than 400° C., solution heat-treating,quenching, tensioning between 2 and 3.5% and ageing.

Patent application US2012/0225271 A1 relates to wrought products with athickness of at least 12.7 mm containing from 3.00 to 3.80% by weight ofCu, from 0.05 to 0.35% by weight of Mg, from 0.975 to 1.385% by weightof Li, wherein −0.3 Mg−0.15Cu+1.65≤Li≤−0.3Mg−0.15Cu+1.85, from 0.05 to0.50% by weight of at least one grain structure control element, whereinthe grain structure control element is selected from the groupconsisting of Zr, Sc, Cr, V, Hf, other rare earth elements, andcombinations thereof, up to 1.0% by weight of Zn, up to 1.0% by weightof Mn, up to 0.12% by weight of Si, up to 0.15% by weight of Fe, up to0.15% by weight of Ti, up to 0.10% by weight of other elements with atotal not exceeding 0.35% by weight.

Application WO 2013/169901 describes alloys comprising, in percentage byweight, 3.5 to 4.4% of Cu, 0.65 to 1.15% of Li, 0.1 to 1.0% of Ag, 0.45to 0.75% of Mg, 0.45 to 0.75% of Zn and 0.05 to 0.50% of at least oneelement for the control of granular structure. The alloys advantageouslyhave a Zn to Mg ratio comprised between 0.60 and 1.67.

There is a need for aluminum-copper-lithium alloy products havingimproved properties compared to those of known products, in particularin terms of compromise between the properties of static mechanicalstrength, in particular the tensile and compressive yield strength andthe properties of damage tolerance, in particular toughness, thermalstability, corrosion resistance and machinability, while having a lowdensity.

In addition, there is a need for a method for manufacturing theseproducts that is robust, reliable and economical.

OBJECT OF THE INVENTION

A first object of the invention is a product based on an aluminum alloycomprising, in percentage by weight, 4.0 to 4.6% by weight of Cu, 0.7 to1.2% by weight of Li, 0.5 to 0.65% by weight of Mg, 0.10 to 0.20% byweight of Zr, 0.15 to 0.30% by weight of Ag, 0.25 to 0.45% by weight ofZn, 0.05 to 0.35% by weight of Mn, at most 0.20% by weight of Fe+Si, atleast one element selected from Cr, Sc, Hf, V and Ti, the amount of saidelement, if selected, being 0.05 to 0.3% by weight for Cr and for Sc,0.05 to 0.5% by weight for Hf and for V and from 0.01 to 0.15% by weightfor Ti, other elements at most 0.05% by weight each and 0.15% by weightin total and the remainder being aluminum.

A second object of the invention is a method for manufacturing a productbased on an aluminum alloy wherein, successively,

-   -   a) a liquid metal bath based on aluminum is prepared comprising        4.0 to 4.6% by weight of Cu; 0.7 to 1.2% by weight of Li; 0.5 to        0.65% by weight of Mg; 0.10 to 0.20% by weight of Zr; 0.15 to        0.30% by weight of Ag; 0.25 to 0.45% by weight of Zn; 0.05 to        0.35% by weight of Mn; at most 0.20% by weight of Fe+Si; at        least one element selected from Cr, Sc, Hf, V and Ti, the amount        of said element, if selected, being from 0.05 to 0.3% by weight        for Cr and for Sc, 0.05 to 0.5% by weight for Hf and for V and        from 0.01 to 0.15% by weight for Ti; other elements at most        0.05% by weight each and 0.15% by weight in total and the        remainder being aluminum;    -   b) a crude form is cast from said liquid metal bath;    -   c) said crude form is homogenized at a temperature comprised        between 450° C. and 550° C. and preferably between 480° C. and        530° C. for a period comprised between 5 and 60 hours;    -   d) said homogenized crude form is hot-worked, preferably by        rolling;    -   e) the hot-worked product is solution heat-treated between 490        and 530° C. for 15 min 125 to 8 h and said solution heat-treated        product is quenched;    -   f) said product is cold-worked with a working of 2 to 16%;    -   g) an ageing is carried out wherein said cold-worked product        reaches a temperature comprised between 130 and 170° C. and        preferably between 140 and 160° C. for 5 to 100 hours and        preferably 10 to 70 hours.

Another object of the invention is an alloy product according to theinvention or that can be obtained according to the method of theinvention, with a thickness comprised between 8 and 50 mm having, atmid-thickness:

-   -   i) a compressive yield strength Rc_(p0.2)(L)≥590 MPa, preferably        Rc_(p0.2)(L)≥595 MPa;    -   ii) a toughness K_(app) (L−T)≥60 MPa√m, preferably K_(app)        (L−T)≥75 MPa √m, with Kapp (L−T) the value of the apparent        stress intensity factor at rupture defined according to standard        ASTM E561 (2015) measured on CCT test specimens of width W=406        mm and thickness B=6.35 mm;    -   iii) a difference between the tensile yield strength Rp_(0.2)(L)        and the compressive yield strength Rc_(p0.2)(L),        Rp_(0.2)(L)−Rc_(p0.2)(L), less or equal to 10 MPa, preferably ≤5        MPa.

Yet another object is an aircraft structure member, preferably anaircraft upper wing skin element.

DESCRIPTION OF THE FIGURES

FIG. 1: Compromise between the toughness K_(app) L-T and the compressiveyield strength Rc_(p0.2) L of the alloys of Example 1.

FIG. 2: Compromise between the toughness K_(q) L-T and the compressiveyield strength Rc_(p0.2) L of the alloys of Example 2.

FIG. 3: Compromise between the compressive yield strength Rc_(p0.2) Land the tensile yield strength R_(p0.2) L for the alloys of Example 2.

FIG. 4: Compromise between the toughness K_(app) L-T and the compressiveyield strength Rc_(p0.2) L of the alloys of Example 3.

DESCRIPTION OF THE INVENTION

Unless otherwise indicated, all indications relating to the chemicalcomposition of the alloys are expressed as a percentage by weight basedon the total weight of the alloy. The expression 1.4 Cu means that thecopper content expressed in % by weight is multiplied by 1.4. Thedesignation of the alloys is made in accordance with the regulations ofThe Aluminum Association, known to the person skilled in the art. Whenthe concentration is expressed in ppm (parts per million), thisindication also refers to a mass concentration.

Unless otherwise indicated, the definitions of metallurgical statesgiven in European standard EN 515 (1993) apply.

The tensile static mechanical features, in other words the ultimatetensile strength R_(m), the conventional yield strength at 0.2%elongation R_(p0.2), and the elongation at rupture A %, are determinedby a tensile test according to standard NF EN ISO 6892-1 (2016), thesampling and direction of the test being defined by standard EN 485(2016). R_(p0.2) (L) means R_(p0.2) measured in the longitudinaldirection.

The compressive yield strength Rc_(p0.2) was measured at 0.2%compression according to standard ASTM E9-09 (2018). Rc_(p0.2) (L) meansRc_(p0.2) measured in the longitudinal direction. The stress intensityfactor (K_(IC)) is determined according to standard ASTM E 399 (2012).The stress intensity factor (K_(Q)) is determined according to standardASTM E 399 (2012). The standard ASTM E 399 (2012) gives the criteriathat allow determining whether K_(Q) is a valid value of K_(1C). For agiven test specimen geometry, the values of K_(Q) obtained for differentmaterials are comparable with each other provided that the yieldstrengths of the materials are of the same order of magnitude.

Unless otherwise indicated, the definitions of standard EN 12258 (2012)apply.

The values of the apparent stress intensity factor at rupture (K_(app))and the stress intensity factor at rupture (K_(c)) are as defined instandard ASTM E561.

A curve giving the effective stress intensity factor as a function ofthe effective crack extension, known as the curve R, is determinedaccording to standard ASTM E 561 (ASTM E 561-10-2).

The critical stress intensity factor K_(C), in other words the intensityfactor which makes the crack unstable, is calculated from the curve R.The stress intensity factor K_(CO) is also calculated by assigning thelength of the initial crack at the beginning of the monotonic load, tothe critical load. These two values are calculated for a test specimenof the required shape. K_(app) represents the factor K_(CO)corresponding to the test specimen which was used to perform the test ofcurve R. K_(eff) represents the factor K_(C) corresponding to the testspecimen which was used to perform the test of curve R.

A mechanical part for which the static and/or dynamic mechanicalproperties are particularly important for the performance of thestructure, and for which a structural calculation is usually required orperformed is here called “structure element” or “structural element” ofa mechanical construction. These are typically elements the failure ofwhich is likely to endanger the safety of said construction, its users,customers or others. For an airplane, these structure elements comprisein particular the elements that compose the fuselage (such as thefuselage skin), the stiffeners or stringers of the fuselage, thewatertight bulkheads, the circumferential frames of the fuselage, thewings (such as the upper or lower wing skin), the stiffeners (orstringers), the ribs and spars and the empennage in particular composedof horizontal and vertical stabilizers, as well as floor beams, seattracks and doors.

According to the present invention, a selected class of aluminum alloyscontaining in particular specific and critical amounts of lithium,copper, magnesium, silver, manganese and zinc allows to preparestructure elements, in particular upper wing skin sheets, having a highcompressive yield strength Rcp_(0.2)(L), a small difference betweencompressive yield strength Rcp_(0.2)(L) and tensile yield strengthRp_(0.2)(L) and a particularly improved apparent stress intensity factorat rupture K_(app). The selected alloy composition of the inventionfurther allows to obtain all or part of the aforementioned advantagesfor a wide range of ageing times (in particular a range of at least 5hours at a given ageing temperature). Such a composition thus allows toguarantee the robustness of the manufacturing method and therefore toguarantee the final properties of the product during industrialmanufacture.

The product based on an aluminum alloy according to the inventioncomprises, in percentage by weight, 4.0 to 4.6% by weight of Cu; 0.7 to1.2% by weight of Li; 0.5 to 0.65% by weight of Mg; 0.10 to 0.20% byweight of Zr; 0.15 to 0.30% by weight of Ag; 0.25 to 0.45% by weight ofZn; 0.05 to 0.35% by weight of Mn; at most 0.20% by weight of Fe+Si; atleast one element selected from Cr, Sc, Hf, V and Ti; other elements atmost 0.05% by weight each and 0.15% by weight in total and the remainderbeing aluminum.

The copper content of the products according to the invention iscomprised between 4.0 and 4.6% by weight, preferably between 4.2 and4.5% by weight and more preferably between 4.2 and 4.4% by weight. In anadvantageous embodiment, the minimum copper content is 4.25% by weight.

The lithium content of the products according to the invention iscomprised between 0.7 to 1.2% by weight. Advantageously, the lithiumcontent is comprised between 0.8 and 1.0% by weight; preferably between0.85 and 0.95% by weight.

The increase in the copper content and to a lesser extent the lithiumcontent contributes to improving the static mechanical strength,however, copper having a detrimental effect in particular on thedensity, it is preferable to limit the copper content to the preferredmaximum value of 4.4% by weight. The increase in the lithium content hasa favorable effect on the density, however the present inventors haveobserved that for the alloys according to the invention, the preferredlithium content comprised between 0.85% and 0.95% by weight allows animprovement in the compromise between mechanical strength (tensile andcompressive yield strength) and toughness. A high lithium content, inparticular above the preferred maximum value of 0.95% by weight, canlead to a degradation of the toughness.

The magnesium content of the products according to the invention iscomprised between 0.5% and 0.65% by weight. Preferably, the magnesiumcontent is at least 0.50% or even at least 0.55% by weight, whichsimultaneously improves static mechanical strength and toughness. Inparticular, for the selected compositions of the present invention, amagnesium content greater than 0.65% by weight can induce a degradationof the toughness.

The zinc and silver contents are respectively comprised between 0.25 and0.45% by weight and 0.15 and 0.30% by weight. Such zinc and silvercontents are necessary to guarantee a compressive yield strength havinga value close to that of the tensile yield strength. In an advantageousembodiment, the products according to the invention have a differencebetween the tensile yield strength Rp_(0.2)(L) and the compressive yieldstrength Rcp_(0.2)(L) less than or equal to 10 MPa, preferably less thanor equal to 5 MPa.

The presence of silver and zinc allows to obtain a good compromisebetween the various desired properties. In particular, the presence ofsilver allows to obtain a product in a reliable and robust manner, thatis to say that the desired compromise in properties is achieved for awide range of ageing times, in particular a time range greater than 5hours, which is compatible with the variability inherent in anindustrial manufacturing method. A minimum content of 0.20% by weight ofsilver is advantageous. A maximum content of 0.27% by weight of silveris advantageous.

A minimum content of 0.30% by weight of zinc is advantageous. A maximumcontent of 0.40% by weight of zinc is advantageous. Preferably, the Zncontent is comprised between 0.30 and 0.40% by weight.

Advantageously, the sum of the Zn, Mg and Ag contents comprised between0.95 and 1.35% by weight, preferably between 1.00 and 1.30% by weight,more preferably still between 1.15 and 1.25% by weight. The presentinventors have observed that the desired optimum compromise inproperties, in particular for elements of the upper wing skin, was onlyachieved for specific and critical values of the sum of Zn, Mg and Ag.

The manganese content is comprised between 0.05 and 0.35% by weight.Advantageously, the Mn content comprised between 0.10 and 0.35% byweight. In one embodiment, the manganese content is comprised between0.2 and 0.35% by weight and preferably between 0.25 and 0.35% by weight.In another embodiment, the manganese content is comprised between 0.1and 0.2% by weight and preferably between 0.10 and 0.20% by weight. Inparticular, the addition of Mn allows to obtain high toughness. However,if the Mn content is greater than 0.35% by weight, the fatigue life canbe significantly reduced.

The Zr content of the alloy is comprised between 0.10 and 0.20% byweight. In an advantageous embodiment, the Zr content is comprisedbetween 0.10 and 0.15% by weight, preferably between 0.11 and 0.14% byweight.

The sum of the iron content and the silicon content is at most 0.20% byweight. Preferably, the iron and silicon contents are each at most 0.08%by weight. In an advantageous embodiment of the invention the iron andsilicon contents are at most 0.06% and 0.04% by weight, respectively. Acontrolled and limited iron and silicon content helps improve thecompromise between mechanical strength and damage tolerance.

The alloy also contains at least one element which can contribute to thecontrol of the grain size selected from Cr, Sc, Hf, V and Ti, the amountof said element, if selected, being from 0.05 to 0.3% by weight for Crand for Sc, 0.05 to 0.5% by weight for Hf and for V and from 0.01 to0.15% by weight for Ti. In an advantageous embodiment, it is selected toadd between 0.01 and 0.15% by weight of titanium. In a preferredembodiment, the Ti content is comprised between 0.01 and 0.08% byweight, preferably between 0.02 and 0.06% by weight. Advantageously inthe embodiments wherein it is selected to add titanium, the content ofCr, Sc, V and Hf is limited to a maximum content of 0.05% by weight,these elements possibly having an unfavorable effect, in particular onthe density and being added only to further promote the production of anessentially non-recrystallized structure if necessary. In a particularlyadvantageous manner, the Ti is present in particular in the form ofparticles of TiC. Against all expectations, the present inventors haveobserved that, in the particular case of the present alloy, the presenceof particles of TiC in the grain refining rod during casting (AlTiCrefining), allows to obtain a product having an optimized compromise inproperties. Advantageously the refiner has the formula AlTi_(x)C_(y)which is also written AT_(x)C_(y) where x and y are the contents of Tiand C in % by weight for 1% by weight of Al, and x/y>4. In particular,the AlTiC refinement in the alloy of the present invention allows animprovement of the compromise between the toughness K_(app) L-T and thecompressive yield strength R_(c)p0.2 L.

The content of the alloy elements can be selected to minimize thedensity. Preferably, the additive elements contributing to increase thedensity such as Cu, Zn, Mn and Ag are minimized and the elementscontributing to decrease the density such as Li and Mg are maximized soas to achieve a density less than or equal to 2.73 g/cm³ and preferablyless than or equal to 2.72 g/cm³.

The content of the other elements is at most 0.05% by weight each and0.15% by weight in total. The other elements are typically unavoidableimpurities.

The method for manufacturing products according to the inventioncomprises the steps of preparation, casting, homogenization, hotworking, solution heat-treating and quenching, tensioning between 2 and16% and ageing.

In a first step, a liquid metal bath is prepared so as to obtain analuminum alloy of a composition according to the invention.

The liquid metal bath is then cast in the form of crude form, preferablyin the shape of a ingot for rolling or an extrusion billet.

The crude form is then homogenized so as to reach a temperaturecomprised between 450° C. and 550° and preferably between 480° C. and530° C. for a period comprised between 5 and 60 hours. Thehomogenization treatment can be carried out in one or more stages.

After homogenization, the crude form is generally cooled to roomtemperature before being preheated in order to be hot-worked. The hotworking can in particular be an extrusion or a hot rolling. Preferably,this is a hot rolling step. The hot rolling is carried out to athickness preferably comprised between 8 and 50 mm and in a preferredmanner between 15 and 40 mm.

The product thus obtained is then solution heat-treated to reach atemperature comprised between 490 and 530° C. for 15 min to 8 h, thenquenched typically with water at room temperature.

The product then undergoes cold working with a working of 2 to 16%. Itcan be a controlled tensioning with a permanent set of 2 to 5%,preferably from 2.0% to 4.0%. In an alternative advantageous embodiment,the cold working is carried out in two steps: the product is first ofall cold rolled with a thickness reduction rate comprised between 8 to12% then subsequently tensioned in a controlled manner with a permanentset comprised between 0.5 and 4%.

The product is then subjected to an ageing step carried out by heatingat a temperature comprised between 130 and 170° C. and preferablybetween 140 and 160° C. for 5 to 100 hours and preferably 10 to 70hours.

The present inventors have observed that, surprisingly, the specific andcritical contents of the alloy of the present invention allow to achieveexcellent properties, in particular a compromise between the compressiveyield strength Rc_(p0.2)(L) and toughness in plane stresses K_(app)particularly improved. Advantageously, these properties can be obtained,for the alloys of the invention, regardless of the ageing time between15 h and 25 h at 155° C., which guarantees the robustness of themanufacturing method.

Advantageously, the granular structure of the products obtained ispredominantly non-recrystallized. The rate of non-recrystallizedgranular mid-thickness structure is preferably at least 70% andpreferably at least 80%.

The products obtained by the method according to the invention, inparticular the rolled products having a thickness comprised between 8and 50 mm, at mid-thickness, have the following features:

-   -   i) a compressive yield strength Rc_(p0.2)(L)≥590 MPa, preferably        Rc_(p0.2)(L)≥595 MPa, with Rc_(p0.2)(L) the compressive yield        strength measured at 0.2% compression according to the standard        ASTM E9 (2018) in the longitudinal direction;    -   ii) a toughness K_(app) (L−T)≥60 MPa√m, preferably        K_(app)(L−T)≥75 MPa√m, with K_(app) (L−T) the value of the        apparent stress intensity factor at rupture defined according to        standard ASTM E561 (2015) measured on CCT test specimens of        width W=406 mm and thickness B=6.35 mm;    -   iii) a difference between the tensile yield strength R_(p0.2)(L)        and the compressive yield strength Rc_(p0.2)(L),        R_(p0.2)(L)−Rc_(p0.2)(L), less than or equal to 10 MPa,        preferably ≤5 MPa.

Advantageously, the features i) and ii) are obtained for a wide range ofageing time, in particular a range of at least 5 hours at a given ageingtemperature. Such a composition thus allows to guarantee the robustnessof the manufacturing method and therefore to guarantee the finalproperties of the product during industrial manufacture.

In an advantageous embodiment, the toughness is such that K_(app)(L−T)≥−0.48 Rc_(p0.2)(L)+355.2, with K_(app) (L−T) expressed in MPa√m,the value of the apparent stress intensity factor at rupture definedaccording to standard ASTM E561 (2015) measured on CCT test specimens ofwidth W=406 mm and thickness B=6.35 mm, and Rc_(p0.2) (L) expressed inMPa, the compressive yield strength measured at 0.2% compressionaccording to standard ASTM E9 (2018).

The alloy products according to the invention allow in particular themanufacture of structure elements, in particular aircraft structureelements. In an advantageous embodiment, the preferred aircraftstructure element is an aircraft upper wing skin element.

These and other aspects of the invention are explained in more detailusing the following illustrative and non-limiting examples.

EXAMPLES Example 1

In this example, plates with a section of 406×1520 mm made of an alloy,the composition of which is given in Table 1, were cast.

TABLE 1 Composition in % by weight of alloys N°1 to 8 Alloy Si Fe Cu MnMg Zn Ti Zr Li Ag 1 0.02 0.03 4.6 0.32 0.62 0.62 0.03 0.13 0.91 0.01 20.02 0.03 4.3 0.31 0.60 0.35 0.03 0.12 0.91 0.24 3 0.03 0.05 4.5 0.340.71 0.04 0.04 0.11 1.03 0.21 4 0.03 0.04 4.3 — 0.33 0.03 0.02 0.15 1.130.21 5 0.03 0.04 4.2 0.33 0.54 — 0.03 0.13 0.88 0.19 6 0.02 0.04 4.40.02 0.21 0.04 0.02 0.14 1.05 0.21 7 0.03 0.04 3.9 — 0.36 — 0.03 0.111.31 0.36 8 0.04 0.06 4.1 0.42 0.42 0.02 0.02 0.15 1.18 0.29

For each composition, the plate was homogenized with a 1st stage of 15 hat 500° C., followed by a second stage of 20 h at 510° C. The plate washot rolled at a temperature above 440° C. to obtain sheets of athickness of 25 mm for alloys 2 to 8 and 28 mm for alloy 1. The sheetswere solution heat-treated at about 510° C. for 3 h, water quenched at20° C. The sheets were then tensioned with a permanent elongationcomprised between 2% and 6%.

The sheets underwent a single-stage ageing as indicated in Table 2.Samples were taken at mid-thickness to measure the static mechanicalfeatures in tension and in compression in the longitudinal direction.The toughness in plane stress was also measured at mid-thickness duringtests of curve R with CCT test specimens 406 mm wide and 6.35 mm thickin the L-T direction. The results are shown in Table 2 and FIG. 1.

The structure of the obtained sheets was mostly non-recrystallized. Therate of non-recrystallized granular mid-thickness structure was 90%.

TABLE 2 Controlled tensile and ageing conditions and mechanicalproperties obtained for the various mid-thickness sheets. PermanentRc_(p0.2) elongation Rp_(0.2) (L) during (L) Com- Kapp controlledTension pression (L-T) Alloy Ageing tensioning (Mpa) (Mpa) (MPa√ m) 1 15h 155° C. 3.0 593 585 71 20 h 155° C. 3.0 604 610 59 2 15 h 155° C. 3.0591 593 76 20 h 155° C. 3.0 601 599 69 25 h 155° C. 3.0 613 63 3 15 h155° C. 3.3 612 607 60 4 15 h 155° C. 3.1 619 614 59 20 h 155° C. 3.1636 637 55 5 20 h 155° C. 3.2 574 570 105 25 h 155° C. 3.2 585 580 79 620 h 155° C. 3.1 628 628 51 7 24 h 150° C. 4.5 606 590 64 8 24 h 150° C.4.0 594 587 72

Example 2

In this example, in addition to the alloy plate 2 of example 1, a platewith a section of 406×1520 mm, the composition of which is given inTable 3, was cast.

TABLE 3 Composition in % by weight of alloys 2 and 10, Alloy Si Fe Cu MnMg Zn Ti Zr Li Ag  2 0.02 0.03 4.3 0.31 0.60 0.35 0.03 0.12 0.91 0.24 100.04 0.02 4.3 0.31 0.64 0.33 0.03 0.14 0.90 0.35

The plates were homogenized at about 510° C. then scalped. Afterhomogenization, the plates were hot rolled to obtain sheets having athickness of 25 mm. The sheets were solution heat-treated for 3 hours atabout 510° C., quenched in cold water and tensioned with a permanentelongation of 3%.

The structure of the sheets obtained was predominantlynon-recrystallized. The rate of non-recrystallized mid-thicknessgranular structure was 90%.

The sheets were tempered between 15 h and 50 h at 155° C. Samples weretaken at mid-thickness to measure the static mechanical features intension, in compression in the longitudinal direction as well as thetoughness K_(Q) in the L-T direction. The test specimens used for thetoughness measurement had a width W=40 mm and a thickness B=20 mm. Theresults obtained are presented in Table 4 and FIGS. 2 and 3.

TABLE 4 Ageing conditions and mechanical properties obtained for thesheets 2 and 10. Difference between Rp_(0.2) (MPa) Com- Tough- intension Tension properties pression ness and RP_(0.2) Ageing Rp_(0.2) Rmproperties K_(Q) (MPa) in time at (L) (L) A Rc_(p0.2) (MPa√m) com- Alloy155° C. (MPa) (MPa) (%) (L) (MPa) L-T pression N°2 10 h 560 598 10 565−5 15 h 591 617 8.3 593 30.6 −2 20 h 601 625 8.5 599 29.9 2 25 h 61327.6 30 h 609 632 7.9 615 −6 N°10 10 h 587 620 10 559 28 15 h 604 6328.5 588 30.7 16 20 h 620 644 8.2 607 25.1 13 25 h 609 24.8 30 h 621 6457.5 609 12

Example 3

In this example, in addition to the plate of alloy 2 of example 1, aplate with a section 406×1700 mm, the composition of which is given inTable 3 was cast using an AlTiC refining (grain refining rod containingnuclei of the TiC type).

TABLE 5 Composition in % by weight of alloys 2 and 9. Alloy Si Fe Cu MnMg Zn Ti Zr Li Ag 2 0.02 0.03 4.3 0.31 0.60 0.35 0.03 0.12 0.91 0.24 90.02 0.04 4.3 0.14 0.61 0.36 0.05 0.13 0.88 0.25

The plates were homogenized at about 510° C. then scalped. Afterhomogenization, the plates were hot rolled to obtain sheets having athickness of 25 mm. The sheets were solution heat-treated for 3 h ataround 510° C., quenched in cold water and tensioned with a permanentelongation of 3%.

The sheets were tempered between 15 h and 25 h at 155° C. Samples weretaken at mid-thickness to measure the static mechanical features intension, in compression in the longitudinal direction as well as thetoughness K_(Q) in the L-T direction. The test specimens used for thetoughness measurement had a width W=40 mm and a thickness B=20 mm. Thevalidity criteria of K_(1C) were met for some samples. Measurements oftoughness in plane stress were also obtained on CCT samples 406 mm wideand 6.35 mm thick. The results obtained are presented in Table 6 and inFIG. 4.

TABLE 6 Ageing conditions and mechanical properties obtained for sheets2 and 9 at mid-thickness Tension properties Rp_(0.2) (L) CompressionToughness Ageing in properties K_(Q) K_(app) time at Rm (L) tension ARc_(p0.2) (L) L-T L-T Alloy 155° C. (MPa) (MPa) (%) (MPa) (MPa ·m^(1/2)) (MPa · m^(1/2)) N°2 15 h 591 617 8.3 593 30.6 76 20 h 601 6258.5 599 29.9 69 25 h 613 27.6 63 N°9 15 h 597 622 9.1 599 28.7 84 20 h603 26.8 80 25 h 602 626 8.5 607 26.9 78

1. A product based on an aluminum alloy comprising, in percentage byweight, 4.0 to 4.6% by weight of Cu, 0.7 to 1.2% by weight of Li, 0.5 to0.65% by weight of Mg, 0.10 to 0.20% by weight of Zr, 0.15 to 0.30% byweight of Ag, 0.25 to 0.45% by weight of Zn, 0.05 to 0.35% by weight ofMn, at most 0.20% by weight of Fe+Si, at least one element selected fromCr, Sc, Hf, V and Ti, the amount of said element, if selected, beingfrom 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% by weightfor Hf and for V and from 0.01 to 0.15% by weight for Ti, other elementsat most 0.05% by weight each and 0.15% by weight in total, the remainderbeing aluminum.
 2. The product based on an aluminum alloy according toclaim 1 wherein the Cu content is comprised between 4.2 and 4.5% byweight, optionally between 4.2 and 4.4% by weight.
 3. The product basedon an aluminum alloy according to claim 1 wherein the Li content iscomprised between 0.8 and 1.0% by weight, optionally preferably between0.85 and 0.95% by weight.
 4. The product based on an aluminum alloyaccording to claim 1 wherein the Zn content is comprised between 0.30and 0.40% by weight.
 5. The product based on an aluminum alloy accordingto claim 1 wherein the Mn content comprised between 0.10 and 0.35% byweight.
 6. The product based on an aluminum alloy according to claim 1wherein the sum of the Zn, Mg and Ag contents comprised between 0.95 and1.35% by weight, optionally between 1.00 and 1.30% by weight, optionallybetween 1.15 and 1.25% by weight.
 7. The product based on an aluminumalloy according to claim 1 wherein the Zr content is 0.10 to 0.15% byweight, optionally between 0.11 and 0.14% by weight.
 8. The productbased on an aluminum alloy according to claim 1 wherein the Ti contentis comprised between 0.01 to 0.15% by weight for Ti, optionally between0.01 and 0.08% by weight, optionally between 0.02 and 0.06% by weight.9. The product based on an aluminum alloy according to claim 8 whereinthe Ti is present in the form of particles of TiC.
 10. A method formanufacturing a product based on an aluminum alloy wherein,successively, a) a liquid metal bath based on aluminum is preparedcomprising 4.0 to 4.6% by weight of Cu; 0.7 to 1.2% by weight of Li; 0.5to 0.65% by weight of Mg; 0.10 to 0.20% by weight of Zr; 0.15 to 0.30%by weight of Ag; 0.25 to 0.45% by weight of Zn; 0.05 to 0.35% by weightof Mn; at most 0.20% by weight of Fe+Si; at least one element selectedfrom Cr, Sc, Hf, V and Ti, the amount of said element, if selected,being from 0.05 to 0.3% by weight for Cr and for Sc, 0.05 to 0.5% byweight for Hf and for V and from 0.01 to 0.15% by weight for Ti; otherelements at most 0.05% by weight each and 0.15% by weight in total andthe remainder being aluminum; b) a crude form is cast from said liquidmetal bath; c) said crude form is homogenized at a temperature comprisedbetween 450° C. and 550° C. and optionally between 480° C. and 530° C.for a period comprised between 5 and 60 hours; d) said homogenized crudeform is hot-worked, optionally by rolling; e) the hot-worked product issolution heat-treated between 490 and 530° C. for 15 min to 8 h and saidsolution heat-treated product is quenched; f) said product iscold-worked with a working of 2 to 16%; g) aging is carried out whereinsaid product reaches a temperature comprised between 130 and 170° C. andoptionally between 140 and 160° C. for 5 to 100 hours and optionally 10to 70 hours.
 11. The product according to claim 1, with a thicknesscomprised between 8 and 50 mm having, at mid-thickness: i) a compressiveyield strength Rc_(p0.2)(L)≥590 MPa, optionally Rc_(p0.2)(L) 595 MPa;ii) a toughness K_(app) (L−T)≥60 MPa√m, optionally K_(app) (L−T)≥75MPa√m, with K_(app) (L−T) the value of the apparent stress intensityfactor at rupture defined according to standard ASTM E561 (2015)measured on CCT test specimens of width W=406 mm and thickness B=6.35mm; iii) a difference between the tensile yield strength R_(p0.2)(L) andthe compressive yield strength Rc_(p0.2)(L), R_(p0.2)(L)−Rc_(p0.2)(L),less than or equal to 10 MPa, optionally ≤5 MPa.
 12. An aircraftstructure element, optionally an aircraft upper wing skin element,comprising a product according to claim 1.